1. Field of the Invention
The present invention relates to an optical device in which an emission layer of a laser light source is slanted to the direction of the reference surface of the light source substrate. Further, the present invention relates to an optical information recording apparatus using the optical device as an optical head.
2. Description of the Related Art
Recently, optical disk drives, such as MO, CD-ROM or DVD drives, have increasingly used an integrated optical head module as the optical head of each optical disk drive. The integrated optical head module is a single module on which both the laser light source that emits a laser light beam to an optical disk and the light receiving element that receives a reflection beam reflected from the optical disk are provided in common.
The use of an integrated optical head module facilitates the positioning of the optical elements in the optical disk drive with accuracy. The use of an integrated optical head module facilitates the manufacture of optical disk drives and allows the small-sized design of optical disk drives. Further, the recording density of optical disks has been increased to a higher level every year. In such circumstances, there is an increasing demand for an optical information recording apparatus using the integrated optical head module that ensures good quality of a reproduced signal obtained from a reflection light.
FIG. 3 is a diagram of a conventional integrated optical head module.
As shown in FIG. 3, the integrated optical head module 11 generally includes a photodetector substrate 21, a sub-mount 22, a semiconductor laser 23, and a reflector mirror 24. The photodetector substrate 21 is constituted by a semiconductor substrate. A plurality of photodetectors 25-1 through 25-8 are disposed on the surface of the substrate 21. A reflected laser beam from an optical disk is divided into plural laser beams, and such laser beams are respectively supplied to the photodetectors 25-1 through 25-7.
The photodetectors 25-1 through 25-4 detect the reflected laser beams from the disk to output tracking error signals. The photodetectors 25-5 and 25-6 detect the reflected laser beams from the disk to output focusing error signals. The photodetector 25-7 detects a reflected laser beam from the semiconductor laser 23 to output a monitor signal that is used to monitor the intensity of the laser light emitted by the semiconductor laser 23. The photodetector 25-8 detects the reflected laser beam from the disk to output an MO signal that is used to generate a reproduced signal.
The monitor signal output by the photodetector 25-7 is supplied to a laser drive circuit of an optical disk drive (not shown). The laser drive circuit controls the intensity of the laser light, which is emitted by the semiconductor laser 23, based on the monitor signal from the photodetector 25-7. The MO signal output by the photodetector 25-8 is supplied to an MO signal detection circuit of the optical disk drive. The MO signal detection circuit generates a reproduced signal through the decoding of the MO signal from the photodetector 25-8.
The tracking error signal and the focusing error signal, which are output by the photodetectors 25-1 through 25-6, are supplied to a focusing/tracking control circuit of the optical disk drive. The focusing/tracking control circuit drives a focusing actuator (not shown) of the optical head in response to the focusing error signal, so that a focusing control of the deflected laser beam on the disk is carried out. The focusing/tracking control circuit drives a tracking actuator (not shown) of the optical head in response to the tracking error signal, so that a tracking control of the deflected laser beam on the disk is carried out.
In the integrated optical head module 11 of FIG. 3, the sub-mount 22 is disposed onto the substrate 21 at a middle position which is slightly deviated from the center of the substrate 21 in a direction “Y1” indicated in FIG. 3. The semiconductor laser 23 is fixed onto the sub-mount 22. The sub-mount 22 isolates the semiconductor laser 23 from the substrate 21, and provides wiring which connects the semiconductor laser 23 to an external circuit.
In the integrated optical head module 11 of FIG. 3, the semiconductor laser 23 is formed from an Al—Ga—As based semiconductor laser chip, and it is fixed onto the sub-mount 22. The semiconductor laser 23 is connected to the laser drive circuit of the optical disk drive. The semiconductor laser 23 emits a laser light beam in response to a drive signal received from the laser drive circuit. The laser light beam is emitted by the semiconductor laser 23 in a direction Y2 indicated in FIG. 3.
The reflector mirror 24 is disposed on the substrate 21 at a middle position on the central axis of the substrate 21. The reflector mirror 24 includes a sloped reflection surface 24a that faces the semiconductor laser 24 in the direction Y1. The sloped reflection surface 24a is substantially at an angle of 45 degrees to the surface of the substrate 21.
The emitted laser beam from the semiconductor laser 23 is incident to the sloped reflection surface 24a of the mirror 24, and the sloped reflection surface 24a reflects the laser beam in the up direction toward the optical disk. The direction of the reflected laser beam is substantially perpendicular to the surface of the substrate 21. The reflected laser beam from the mirror 24 is divided by optical elements of the optical head into plural laser beams, and most of such laser beams are directed to the optical disk 10 but one of such laser beams is directed to the photodetector 25-7.
FIG. 4A and FIG. 4B are diagrams for explaining a relationship between the reflected light beam and the reflector mirror in the conventional integrated optical head module. FIG. 4A is a perspective view of the mirror 24, and FIG. 4B is a top view of the mirror 24.
The semiconductor laser 23 is provided with an emission 35 layer that is parallel to the surface of the substrate 21. The semiconductor laser 23 is disposed such that the optical axis of the emission light beam from the semiconductor laser 23 is in the direction Y2 that is parallel to a radial direction of the optical disk 10. The reflector mirror 24 is disposed such that the sloped reflection surface 24a is at an angle of 45 degrees to the surface of the substrate 21.
The emitted laser beam from the semiconductor laser 23 has a given direction of polarization with respect to the surface of the substrate 21 as indicated by the arrow X in FIG. 4A and FIG. 4B.
The reflector mirror 24 is disposed such that the reflection surface 24a is substantially parallel to the direction of polarization of the emitted laser beam. Hence, the semiconductor laser 23 and the reflector mirror 24 are disposed on the substrate 21 such that the direction of polarization (or the direction X) of the reflected laser beam from the mirror 24 accords with a tangential direction (or a track direction) of the optical disk 10.
Generally, a laser beam emitted from an emission layer of a semiconductor laser has an elliptic distribution of intensity, the ellipse having a major axis parallel to the lateral direction of the emission layer and a minor axis parallel to the longitudinal direction of the emission layer. It is desired that a laser beam emitted by a semiconductor laser used in a magneto-optical disk drive provide a circular distribution of intensity. For this purpose, Al—Ga—In—As—P based semiconductor lasers have been proposed, which are configured to provide an emitted laser beam having a nearly circular distribution of intensity. A high-output S3 (self-aligned stepped substrate) semiconductor laser has been developed as one of such Al—Ga—In—As—P based semiconductor lasers.
FIG. 5 shows a structure of the Al—Ga—In—As—P based S3 semiconductor laser.
As shown in FIG. 5, in the Al—Ga—In—As—P based S3 semiconductor laser 30, a substrate 31, a clad layer 32, a strain-MQW activation layer 33, a first clad layer 34, a current block layer 35, a second clad layer 36 and a contact layer 37 are provided.
When producing the Al—Ga—In—As—P based S3 semiconductor laser 30, selective etching of the substrate 31 having a primary surface of (100) 6° off (which will be called the reference surface) is first performed. As a result of the etching, the (411)A surface is exposed on the substrate 31 on which a p-type emission layer 38 is formed. The clad layer 32 and the activation layer 33 are formed on the substrate 31 having the (411)A surface. In the activation layer 33, the p-type emission layer 38 is provided on the (411)A surface, and an n-type region 39 is provided on the (100) 6° off reference surface. The current block layer 35 is formed on the (100) 6° off reference surface only, which provide the current blocking function.
As described above, the Al—Ga—In—As—P based S3 semiconductor laser 30 is configured to provide an emitted laser beam having a nearly circular distribution of intensity. However, in the Al—Ga—In—As—P based S3 semiconductor laser 30, the p-type emission layer 38 is formed on the (411)A surface, and the substrate 31 has the (100) 6° off reference surface. For this reason, the emitted laser beam from the emission layer 38 of the semiconductor laser 30 is slanted to the direction of the reference surface of the substrate 31, and the slanted angle (“θ” indicated in FIG. 5) of the emission layer 38 is about 13.5 degrees to the direction of the reference surface of the substrate 31. Therefore, in the conventional integrated optical head module that uses the Al—Ga—In—As—P based S3 semiconductor laser 30 as the laser light source, the direction of polarization of the emitted laser beam from the emission layer 38 is slanted to the direction of the reference surface of the substrate 31 and does not accord with the tangential direction (or the track direction) of the optical disk 10.
FIG. 6A and FIG. 6B are diagrams for explaining laser light emission of the conventional integrated optical head module using the Al—Ga—In—As—P based S3 semiconductor laser 30 as the laser light source. FIG. 6A is a perspective view of the mirror 24, and FIG. 6B is a top view of the mirror 24.
As shown in FIG. 6A and FIG. 6B, even when the conventional integrated optical head module uses the Al—Ga—In—As—P based S3 semiconductor laser as the laser light source 23, the semiconductor laser 23 and the reflector mirror 24 are disposed such that the optical axis (indicated by “C11” in FIG. 6A) of the laser emission of the semiconductor laser 23 is parallel to the central axis (indicated by “C12” in FIG. 6A) of the photodetector substrate 21, and the direction of the reflected laser beam from the sloped reflection surface 24a is substantially perpendicular to the surface of the substrate 21.
However, in the conventional integrated optical head module, the direction of polarization of the emitted laser beam from the semiconductor laser 23 is slanted to the direction of the reference surface of the substrate of the semiconductor laser 23 (the slanted angle is 13.5 degrees). When the semiconductor laser 23 is disposed on the substrate 21 as shown in FIG. 3, the direction of polarization (indicated by “D11” in FIG. 6A) of the emitted laser beam from the semiconductor laser 23 is slanted to the optical axis (indicated by “C11” in FIG. 6A) of the laser emission.
As shown in FIG. 6B, the emitted laser beam from the semiconductor laser 23 is incident to the reflection surface 24a of the mirror 24 with the direction of polarization (indicated by “D12” in FIG. 6B) is slanted. Hence, the direction of polarization (indicated by “D13” in FIG. 6A) of the reflected laser beam from the reflection surface 24a is slanted to or rotated from the desired polarization direction (indicated by “D10” in FIG. 6A).
Therefore, in the conventional integrated optical head module, the direction of polarization of the emitted laser beam from the semiconductor laser 23 is slanted to the direction of the reference surface of the semiconductor laser substrate, and if the semiconductor laser 23 is disposed on the substrate 21 as shown in FIG. 3, the direction of polarization “D13” of the reflected laser beam from the reflection surface 24a of the mirror 24 is rotated from the desired polarization direction “D10”. Hence, the distribution of intensity of the reflected laser beam is changed due to the slanted emission layer, and it is difficult to attain good quality of a reproduced signal derived from the reflected laser beam from the optical disk when the semiconductor laser 21 is disposed as shown in FIG. 3.
A conceivable method for eliminating the problem of the conventional integrated optical head module is to perform the design change of the optical elements, including the sub-mount 22, such that the modified optical elements are suited for the requirements of the conventional integrated optical head module using the Al—Ga—In—As—P based S3 semiconductor laser 30. However, the general-purpose optical elements cannot be used for the conventional integrated optical head module, and the manufacturing cost is increased if the above method is used. Further, even if the modified optical elements are used at the sacrifice of the manufacturing cost, it is difficult to immediately attain good quality of a reproduced signal derived from the reflected laser beam from the optical disk.